Field emission device with microchannel gain element

ABSTRACT

A field emission device with a micro-channel gain element included a plurality of field emission or “cold” cathodes ( 102 ) formed into an array. The cold cathodes are typically modulated by a grid ( 108 ) having a driving voltage. A microchannel gain element ( 114 ) is secondary electron emissive material within each of the channels. The channels correspond number and location to the cathode and enable multiplication of electron emitted by the cathodes. Multiplication of the electrons enables the cathodes to be driven at a lower current of emitted electrons than normally applied, absent the microchannel, to obtain the same resulting beam. The beam existing each of the micro-channels is directed to an anode ( 130 ), which can include a phosphor ( 128 ) for use in a flat panel display. Alternatively, anode can include a semiconductor substrate having a reactive resist, and an electrostatic lens structure can be employed to focus the beams to produce a mask pattern on the substrate according to a predetermined mask pattern.

RELATED APPLICATION

This is a U.S. national application of PCT/US98/05127, filed Mar. 16,1998, which is a continuation-in-part of copending U.S. patentapplication Ser. No. 08/416,078, filed Apr. 4, 1995, now U.S. Pat. No.5,849,333, which issued Dec. 15, 1998.

FIELD OF THE INVENTION

This invention relates to field emission devices and more particularly afield emission device utilizing a microchannel gain element to multiplyelectron emission.

BACKGROUND OF THE INVENTION

It has become increasing common in the construction of image-generatingdevices to utilize a field emission device or “cold” cathode as a sourceof electrons for exciting a surface that generates a visible light. Thecathode is placed in a vacuum enclosure and electrons are emitted fromthe cathode by action of a strong electric field adjacent the cathode.The electric field results from the cathode's geometry and from the useof a collector or anode adjacent the cathode. The anode is biased with astrong positive electric potential relative to the cathode. The emittedelectrons generate a beam that passes between the cathode and anode.This beam can be modified by a third electrode known as a gate or grid.Further electrodes can also be added within the vacuum space between thecathode and the gate to further modify the electron beam. The resultingassembly can be termed a triode, tetrode, pentode, or the like,depending upon the number of electrodes present, in addition to theanode and cathode.

A generalized field emission device according to the prior art isdetailed schematically in FIG. 1. The cathode 20 includes an emittersection 22 having a sharp emitter point 23. The emitter 22 is locatedbehind a gate 24 having an opening 26 through which electrons pass tostrike an anode 28. The cathode 20, gate 24 and anode 28 are separatedfrom each other spatially and are enclosed in an evacuated envelope 30.Field emission or “cold” cathodes such as cathode 20 are known generallyto those who are skilled in the art. Current methods of fabricating andcharacterizing these devices is described in a special issue of theJournal of Vacuum Science and Technology B. Microelectronics andNanometer Structures, Second Series, vol. 12, no. 2. March/April 1994andis hereby expressly incorporated herein by reference.

When the anode 28 of FIG. 1 is employed as a display device (such as ina flat panel display) a phosphor is provided integrally to the anode.The phosphor is sensitive to electron excitation and emits visiblelight. Likewise, the cathodes 20 are organized in arrays that areaddressable to generate an image. Where the structure of FIG. 1 is to beemployed in electron microscope, the object to be analyzed is part ofthe anode 28.

In a display device, the brightness of light emitted by the phosphorscreen depends upon the density of electrons that strike a given area ofthe screen and the energy derived from the voltage difference betweenthe anode and the cathode. The brightness also depends upon theefficiency of the phosphor in converting the energy of electrons tophotons of visible light. Typically, to achieve a high current density,and therefore a high-brightness in a display device the electric fieldthat extracts electrons at the emitter 22 must be in the range of 10⁷Volts/centimeter. This high field strength invariably leads to cathodetip damage by erosion. Such erosion makes the emitted beam unstable inthe short term and impairs long-term reliability. Thus, erosion makesconstruction of flat panel displays using the structure FIG. 1 limitedin brightness. Cathode damage and beam instability are also encounteredin developing electron sources for integrated circuit lithography andelectron microscopy.

IBM Research Report RC19596 (86076), Jun. 3, 1994, entitled “EmissionCharacteristics Of Ultra-Sharp Cold Field Emitters” (now published inthe Journal of Vacuum Science & Technology. B12(6), p.3431. Nov./Dec.1994) by Ming L. Yu, et al describes cathode deterioration at highcurrent densities due to ohmic power dissipation, electron static stressand ion etching from residual gases in the evacuated space. The reportalso relates to long term and short term current instabilities duringelectron emission at high current densities.

It is desirable that electrons emitted by each cathode be asconcentrated as possible when used in a display device. Transversespreading of electrons generates larger pixels and, thus, lowers thedensity of pixels on the screen. Spreading of the electron beam alsoresults in noise and reduced contrast, since electrons strike thephosphor in an area other than the intended pixel. To combat spreading,displays according to the prior art have located the anode/phosphorscreen as close to the cathode array as necessary to attain desiredresolution, contrast and signal-to-noise ratio. However, the placementof the anode and cathode in close proximity makes construction difficultand imposes design constraints that increase construction cost and/ordegrade performance.

While the cathode structure FIG. 1 can also be utilized in a colordisplay device, by providing phosphors of three different colors (red,green and blue, for example) in a cluster with three independentemitters, the above-described problems remain. Additionally, accuratecontrol of color generated by the three, clustered, subpixels must alsobe maintained. Electron beam spread and instability complicate themaintenance of good color fidelity.

In order to overcome the problems inherent in a field emission cathodeoperating at high current, it is contemplated that the cathode can beoperated at a substantially lower current. However, a lower current,while reducing erosion and increasing electron beam stability, does notgenerate a beam of sufficient density. The resulting weak beam istypically insufficient to cause currently available phosphors to emit abright visible light. The emitted electron beam is also insufficient toperform detailed electron microscopy or electron beam lithography withimproved performance and results.

In the field of lithography for producing integrated circuits, inparticular, the goal is to create complex, microscopic circuit patternson a semiconductor wafer substrate composed, for example, of silicon orgallium arsenide. To accomplish the patterning process, the wafer isfirst covered with a protective layer of polymeric, light-sensitivematerial, known as a photoresist. The photoresist is dried by a bakingor “curing” process, and then is exposed selectively to light in apattern defined by a “mask” having transparent and opaque areasanalogous to the desired circuit pattern. Where the mask is transparent,light passes through, onto the substrate, exposing each portion of thecircuit pattern. The exposed circuit pattern on the substrate undergoesa chemical change in response to the light, while the remainingunexposed photoresist is unchanged chemically. When the photoresist is“developed,” in a manner similar to photographic film, a defined circuitpattern on the substrate at a microscopic level results. The developedphotoresist enables selective processing of the semiconductor wafer toproduce a layered structure with a variety of conducting, semiconductingand insulating media that define the finished circuit.

A mask can be placed directly on the surface of the substrate to producea 1:1 scale image on the substrate. This is termed “contactlithograplhy.” since the mask essentially contacts the substrate.Alignment of the mask is important in a contact arrangement, and aspecial contact aligner is used for this purpose, since the mask mustmaintain alignment with the substrate within a close tolerance range asthe substrate undergoes several different layers of lithography.

Other techniques for producing circuit patterns on a substrate entailthe use of “proximity alignment,” “projection alignment,” or“step-and-repeat alignment.” The most widely employed in thesemiconductor industry is currently the step-and-repeat technique. A“stepper” mechanism transfers the image from a relatively larger 5× or10× scale (for example) mask to the substrate using, a reduction processthat employs a sophisticated optical system. The substrate is moved inincrements, or “stepped,” under the optical system as the processproceeds incrementally to expose the entire wafer. Typically,lithography is one of the slowest steps in the fabrication of wafers.The wafer throughput of a current stepper is on the order of one hundredwafers per hour.

In each of the above-described photoresist exposure techniques, visibleor ultraviolet light is generally used to as the exposure agent. Thewavelength of light poses a general limit on the width of circuit lineson an exposed substrate. In general, line width of no less than 180 nmcan be produced using light in the ultraviolet region of the spectrum.Conversely, electron beams can be focused to a much-finer extent. Usingan electron beam, lines having a width of 20 nm or less can be achieved,essentially an order of magnitude finer than that possible withphotolitlhographic processes. This enables much denser packing oftransistors onto a given area enabling higher performance in a smallerpackage.

Conventional electron beam lithography techniques are limited in thatthey require the substrate to be exposed by the beam in a serial fashionby scanning the beam over the substrate, typically in a line-by-line(raster-style) manner. The scanning time involved is significantlylonger than the step-and-repeat (stepper) method using visible orultraviolet light.

In order to improve the throughput of electron beam lithography,research has been conducted by several investigators using arrays ofparallel electron beams. The source of the electrons is typically anarray of electron emitters. Such emitters have generally comprised“cold” cathodes, since hot cathodes (heated wires) or “warm” cathodes(Schottky emitters) generate excess heat when closely packed together.With a sufficiently large array of emitters and a corresponding array ofelectron beam lenses to focus the output of the emitters it has beenpossible to increase the throughput of patterned semiconductor waferssubstantially.

In operation, the array of emitters is matrix-addressable. That is, agiven field emission device in the array can be individually addressedvia row and column drivers. A typical field emitter consists of acathode tip(s) and a gate or “grid.” When a particular row and column inthe array are energized by the application of a suitable voltage, theemitter at the intersection of the row and column addresses isactivated, and emits electrons. The electron beam lens corresponding, tothe location of the emitter focuses the emitted electrons onto an anode,typically with an energy of 1,000 to 100,000 electron Volts. In thisexample, the anode is a wafer substrate coated with a resist that issensitive to the impacting electron beam. Thus, a focused spot patternis formed on the wafer. If the wafer is moved relative to the incidentelectron beam, then a focused line is patterned on the resist of thewafer. Likewise, each emitter can independently pattern a line in thesubstrate. It follows that, if two adjacent emitters in a row areactivated simultaneously, then the motion over the spacing between theemitters produces a line twice as long as that produced by a singleemitter. Likewise, if all the emitters in a row are activatedsimultaneously a line is patterned in the substrate that corresponds tothe full length of the row of emitters even though the total motion toproduce the line is limited only to the distance between emitters. Inthis parallel method of patterning the wafer it is hence, possible toimprove the wafer throughput substantially beyond what is possible inthe serial method described above.

However, in order to achieve sufficient electron beam current to exposethe pattern in the resist-coated wafer, the field emission cathodes ofthe prior art are operated at current levels that cause short-terminstability and poor long-term reliability. Thus, while the parallelmethod of electron beam lithography is attractive for its potentiallyhigh throughput, and for obtaining linewidths not possible withphotolithography it is limited in application by significant reliabilityconsideration—considerations that clearly do not lend the technique tomass-production semiconductor fabrication processes.

Accordingly, it is an object of this invention to provide a fieldemission device structure that enables operation of a field emissioncathode at a lower current without the corresponding loss in electronbeam strength. The structure should be capable of tightly focusing thegenerated beam even when the anode is located at an increased distancefrom the cathode. In a display, the cathode should generate a beam thattriggers sufficient visible light emission in conventional phosphors.The generated beam should be stable and the cathode should haveincreased reliability due to reduced erosion. In electron beamlithography, the generated beam or beams should enable rapid and finelyfocused exposure of a circuit pattern on a semiconductor substrategenerally coated with a resist that is reactive to the impactingelectrons.

SUMMARY OF INVENTION

A field emission device, with a microchannel gain element according tothis invention, provides an array of field emission or “cold” cathodeslocated on a substrate. The cathodes can be addressed individually or ingroupings that correspond, in an image display device, to pixels. A gateor grid system is typically located in conjunction with the cathodearray and is driven at a constant voltage with a superimposed modulatingvoltage to control emission of the cathodes. The cathodes are located inan evacuated space so that, upon application of a predetermined voltage,an electric field enables emission of electrons from individuallyaddressed cathode emitters. The emitted electrons pass through the gateor grid and, according to this embodiment, enter a microchannel gainelement. The microchannel gain element includes a pair of opposing anodesides that are driven at a voltage difference. The microchannel gainelement also includes a plurality of microchannels that correspond toeach of the cathodes. The microchannels include asecondary-electron-emissive layer therein. When electrons from each ofthe cathodes strike the emissive layer, the emissive layer generatesadditional electrons. A cascade effect ensues as electrons pass downeach of the microchannels, and the resulting electron beam that exitseach of the channels typically has a gain in a range of 100-200 (ormore) relative to the entering electron stream. The use of a gainelement enables the current generated by each of the cathodes to besubstantially lower for a given anode current. This lower current addsto electron beam stability and reduces erosion of cathode emitters. Asubstantially conventional anode is located adjacent the exit of themicrochannel structure.

The anode, in an image device, can comprise a glass, or othertransparent material, plate having a phosphor and a thin metallic filmthereon. Alternatively, in an electron microscope, the anode can includethe object being viewed. In a lithographic process, the anode caninclude a semiconductor wafer coated with a sensitive resist. Inparticular, the anode can comprise a semiconductor substrate having areactive resist, and an electrostatic lens structure can be employed tofocus the beams to produce a pattern on the substrate according to apredetermined geometric design. The pattern is provided to an addresscontroller that selectively activates the emitters and gating structure,or “grid,” to reproduce the mask pattern in a single pass, or a sequenceof passes. The lens structure can comprise a plurality of plates drivenby predetermined voltages and stacked with apertures aligned with theemitters and outlets of the microchannel plate. A plurality of stackedmicrochannel plates can be provided with channels aligned to furtherincrease the gain of the beam entering the lens structure.

In all of the above-described embodiments, the use of a microchannelgain element enables generation of more-stable electron beams andincreases cathode reliability and life. The microchannel plate accordingto this invention can be formed in a variety of ways from a variety ofmaterials. The construction lends itself to improved electron beamlithographic processes and enables the formation of high resolutionpixels and/or circuit pathways.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the invention willbecome more clear with reference to the following detailed descriptionas illustrated by the drawings in which:

FIG. 1 is a schematic cross-section of a field emission device accordingto the prior art;

FIG. 2 is a schematic side view of a field emission device including amicrochannel (lain element according to this invention;

FIG. 3 is a schematic cross-section of the microchannel gain element ofFIG. 2;

FIG. 4 is a more detailed partial schematic cross-section of themicrochannel gain element of FIG. 3 illustrating the substitute layersthereof;

FIG. 5 is a schematic diagram of an array of field emission cathodesaccording to this invention;

FIG. 6 is a voltage and timing diagram relative to the array of FIG. 5;

FIG. 7 is a schematic cross-section of a field emission device with amicrochannel gain element installed in a flat panel display according tothis invention;

FIG. 8 is a schematic plan view of a grouping of primary color subpixelsfor use in a color display according to this invention;

FIG. 9 is a schematic side view of a microchannel electron beamlithographic device according to an alternate embodiment of thisinvention;

FIG. 10 is a partial schematic plan view detailing the two-dimensionalarrangement of the lithographic device of FIG. 9; and

FIG. 11 is an exploded perspective view of an electron beam lithographicdevice according to this invention for use in the manufacture ofsemiconductor circuits.

DETAILED DESCRIPTION

FIG. 2 illustrates a basic embodiment of a field emission device withmicrochannel gain element according to this invention. The cathode 40includes an emitter structure 42 formed conventionally according to thisinvention. A conventional gate structure 44 having an opening 46 isprovided in conduction with and adjacent the cathode 40. Some examplesof field emission device cathodes, typically containing sharply tippedpoints, are structures formed from metal such as molybdenum, nickel ortungsten deposited on, for example, glass or silicon. Diamond canpossibly be employed, and materials such as gallium nitride and metalcarbides have been investigated as possible cathode structures.

The cathode includes an emitter tip 43 that is pointed to generate avery high local electric field. Shaping of the tip 43 is accomplished byetching and/or depositing a tip of material or forming a thin ridge witha sharp edge on the active region. Cathodes will emit electrons in avacuum when excited with grid voltages ranging from less than 10 Volts(the Matsushita Tower Cathode, for example) to approximately 150 Volts.

The emission voltage depends largely upon materials and geometry. Theelectric field requisite for emission is approximately 10⁷ Volts/cm. Ananode 47 is provided remote from the cathode 40. In a display device,the anode is typically a thin conducting film such as vapor-depositedaluminum. As noted above, the anode can include a phosphor for emittingvisible light in a display device embodiment. Alternatively, the anode47 can comprise an object being viewed through an electron microscope ora lithographic target. By “anode” it is meant a display screen, alithographic target including a semiconductor wafer substrate, amicroscopic object or any other structure intended to selectivelyreceive beams of electrons. It can be assumed that a plurality ofcathodes 40 and emitters 42 are provided in, for example, a paneldisplay. Tile entire structure is surrounded by a vacuum envelope 48that can comprise glass or a similar sealed structure. Between the grid44 and the anode 47 is located a microchannel structure 50. It is themicrochannel that enables multiplication of electrons emitted by thecathode emitter 42 according to this invention.

The microchannel 50 includes a series of through passages such aschannel 52. Each channel is located adjacent an emitter 42 so thatemitted electrons arc aligned with the channel 52. The channel accordingto this invention is typically in the range of 0.1 to 1 millimeter inlength and approximately 2 to 30 microns in diameter. The channel can beconstructed with a body or substrate 54 of silicon, glass, metal (suchas aluminum) or another suitable base material. Note that, if the bodyis conductive, it should be suitably electrically isolated as showngenerally in FIG. 4. The front and rear surfaces of the channel,adjacent the grid 44 and the anode 47 are coated with conductiveelectrodes 56 and 58, respectively. The coating typically consists of aconductivity modifier, dopant or vapor-deposited material applied to thebody material 54. The construction of a microchannel display accordingto this invention is described further below.

Within the surface of the microchannel 52 is disposed a secondaryelectron emissive layer 60 according to this invention. The electrodes56 and 58 are driven at a constant voltage potential (between eachother) ranging from approximately 600 to 1000 Volts according to thisembodiment. The driving of the microchannel 50 at this voltage willgenerate a gain in the range of approximately 100 to 200 times.

Operation of the microchannel 50 is detailed more particularly in FIG. 3An electron 62 enters the channel 52 along a path 64 shown as a dottedline. When the electron 62 comes into contact with the secondaryelectron emissive layer 60 (sometimes referred to as a dynode due to theanalogous dynode structure in a photomultiplier tube), the electroncauses the emission of a plurality of electrons that, typically, projectalong a series of paths 66 to each strike tile emissive layer 60, yetagain. Each emitted electron that strikes the emissive layer 66 againgenerates a further multiplicity of emitted electrons forming acontinuously cascading group of electrons that finally exits the channel52 as a multiplied electron beam 68. As noted, this multiplication canbe substantially more than 100 to 200 times. The resulting beam 68 isformed of a large number of energized electrons that according to thisembodiment, are sufficient to cause a phosphor to glow brightly as thebeam's electrons are collected at the anode 47.

A practical application of a microchannel 50 according to thisembodiment entails the use of an array of such microchannels, known asmicrochannel plates. Microchannel plates were originally developed forimage intensifier tubes in night vision application. Because of the(rain generated by the microchannel plate, the anode current can be mayorders of magnitude greater than the current emitted by the cathode. Thegain can be as much as 10⁸. Hence, low-light images focused on aphotocathode are transferred to an anode/screen with a much higherintensity. In a night vision application, the source of electrons is aphoto-sensitive cathode rather than a field emission cathode asdescribed herein. Light incident upon the photo-sensitive cathode causesthe emission of electrons, usually from a surface that is speciallycoated with a material having a negative electron affinity. Thelow-intensity image that originates at the cathode is, thus, transferredto the viewing, screen after intensification resulting fromamplification in the microchannel plate.

Microchannel plates in current use have typically been fabricated fromarrays of microscopic glass cylinders with a channel density of between10⁵ to 10⁷ per square centimeter. Galileo Electro-Optics Corporation isa manufacturer of glass microchannel plates. Details related to theGalileo microchannel plates are described by J. J. Fijol. et al in“Secondary Electron Yield of SiO₂ and Si₃N₄ Thin Films for ContinuousDynode Electron Multipliers.” Applied Surface Science 48/49, pp. 464-471(1991). Fabrication techniques for these microchaniel plates has beendisclosed in U.S. Pat. Nos. 5,086,248 and 5,205,902, which are expresslyincorporated by reference herein. Galileo has most recently developed amicrochannel plate based upon silicon rather than glass. Details of thesilicon plate's fabrication are reported under NIST ATP Contract Number70NANB3H1371. Fabrication details for a silicon microchannel plate arealso reported in “Characteristics and Applications of AdvancedTechnology Microchannel Plates” by J. R. Horton, et al. in SPIE, vol.1306, Sensor Fusion III (1990). These documents are also expresslyincorporated herein by reference.

Construction of a microchannel plate according to this inventiontypically entails the slicing, grinding and fine polish of a wafer ofsubstrate material. The wafer is subsequently coated with a photoresistand exposed by photo lithography to generate a series of microscopicholes in the photoresist along a surface of the wafer. The holes arethen exposed to a chemical etching process that etches away thesubstrate material within the confines of each hole to form athrough-channel. A conductive layer is then formed over the surfaces ofthe wafer and in each channel. This conductive layer is applied,typically, by vapor deposition. Subsequently, an electron emissive layeris applied into the channels. As a final step, a metal surface isevaporated onto the opposing sides of the wafer to generate the dynode.Each wafer is typically inspected and tested prior to installation in apredetermined device.

With reference to FIG. 4, it is contemplated that silicon can be used asa substrate 70 for a microchannel plate 50 according to this embodiment.Since silicon acts as a conductor even when it is nearly intrinsic, ahigh voltage across the length of the channel 52 would draw very highcurrent and adversely affect operation of the microchannel plate 50.Accordingly, the underlying silicon substrate 79 is isolated from theapplied voltage by an insulator that, in this embodiment, comprises anoxynitride film 72. Since SiO₂ and Si₃N₄ are both good secondaryelectron emitters, this film 72 could act as an electron emitter.However, if there is no continuous conducting path for the current path,the secondary emission current cannot be sustained and no amplificationwould occur. Thus, a thin conducting film 74, which can compriseamorphous silicon or another material, is deposited on the oxynitride 72within the channel 52. This layer 74 provides a resistive bridge betweenthe two dynodes while the oxynitride layer 72 insulates both the dynodesand the resistive “bridge” layer from the silicon substrate 70. Notethat the lower insulating layer is typically unnecessary when thesubstrate comprises an insulator, such as glass.

A further layer of emissive oxynitride 76 is then deposited over theconductive layer 74. A high voltage is generated along the length of theresistive layer 74 due to its continuity with the deposited endconductors 56 and 58. The specific design of the conducting film 74 isgenerally specific to the microchannel plate application. For example,in an image intensifier, it is desirable that the anode current compriseonly a small fraction of the current carried by the resistive film inthe channel of the microchannel plate. In other words, the conductanceof the film should be relatively high. High conductance avoids anynon-linearity in the response of the microchannel plate and assuresfaithful reproduction of the object viewed to a resulting image. In afield emission device application, however, it can be desirable to avoidexcess (parasitic) current in the resistive film to minimize powerdissipation. As such, the resistive film 74 should typically exhibitlower conductance so that at least some saturation of the output currentmay occur.

FIGS. 5, 6 and 7 illustrate a specific implementation of theabove-described field emission device with microchannel gain in a flatpanel display. With reference to FIG. 7, the cross-section of a singleemitter and channel is shown. It can be assumed that a practical flatpanel display according to this invention would comprise a large arrayof such emitter and channels all located within an evacuated space. Thedetails of such a display are described further below. The emitterstructure detailed in FIG. 7 can comprise a single pixel of the display.Where color is utilized, it is desirable to have three individualsubpixels that direct an electron beam onto three distinct phosphorsthat glow (for example) red, green and blue, respectively. It iscontemplated that each subpixel can, in fact, consist of many emittersthat each include a corresponding microchannel element. Thus, the term“pixel” as used herein is meant to include a plurality of constituentsubpixels and/or emitter and microchannel structures. Furthermore, anemitter can be constructed from a plurality of cathode points allemitting electrons to a single microchannel. The term “emitter” orcathode should, therefore, be used broadly to define one or moreindividual tips in a cluster feeding a single aligned microchannel.

As described further below, with reference to addressing, each redundantemitter for a given subpixel is electrically connected in parallel.Similarly, for a monochromatic display, each monochromatic pixel can beformed from many emitters in parallel. It should be clear that oneadvantage of forming a single pixel from a plurality of emitters is thatthe failure of any one, or group, of emitters will not generally renderthe pixel inoperative.

FIG. 7 details a generic design for a single pixel or subpixel in a flatpanel display according to this invention. The cathode 100 includes anemitter 102 that in this embodiment includes a pointed emitter tip 104.The emitter 102 is disposed on a substrate 106 that can comprise a platehaving a plurality of cathode emitters 102 formed thereon. It iscontemplated that each of the cathode emitters 102 is independentlyaddressable by interconnections formed within the substrate 106 using,for example, lithographic circuit-forming techniques. As discussedabove, a variety of known “cold” cathode structures can be usedaccordions to this invention. The difference between currently usedfield emission devices and the field emission device contemplatedaccording to this invention is that the current derived from the cathodecan be reduced by at least two orders of magnitude due to theamplification provided by the microchannel element. A grid 108 having agrid opening (gate) 110 is provided at the front of the cathode with theopening 110 aligned with the emitter 102. The grid facilitatesmodulation of the current from the cathode. Typical grid voltages canvary from 8 Volts to 140 Volts. The grid typically is biased at aconstant voltage with a superimposed modulating voltage to achieve adesired emission of electrons from the cathode 102. The exact voltageused to drive the grid depends upon the performance of the cathode 106and the gain obtained by the microchannel, as well as the sensitivity ofthe phosphor (so that an appropriate brightness is achieved for a givencathode current). A low grid voltage can enable the use of low-voltageCMOS drivers to drive the system and thus, can be preferable from amanufacturing and power consumption standpoint.

A spacer 112 is positioned between the grid 108 and the microchannelelement 114 according to this invention. The thickness of the spacer 112is a design option. The spacer 112 includes a channel 116 that enableselectron flow from the cathode into the microchannel of the microchannelelement 114. The spacer 12 can comprise any suitable insulatingmaterial. Another option is to eliminate the spacer and operate the gridand the first dynode at the same voltage.

The microchannel element 114 includes opposing conductive dynode plates120 and 122. A constant voltage differential of between 600 and 1000Volts is applied between the plates 120 and 122. The associated gain forthe channel 118 will then be in the range of 100 to 200. A furtherspacer is provided adjacent plate 122. This spacer 126 is, again, adesign option, but can be constructed of any appropriate insulatingmaterial. Adjacent to the spacer 126 is the anode structure 128. Theanode 128 includes a phosphor element 130 that glows visibly in responseto electron excitation. The phosphor element 130 is disposed upon atransparent plate 132 that can comprise glass according to thisinvention.

The construction of the phosphor element 130 and glass screen 132 can beidentical to that of any conventional flat panel display. Industry iscurrently experimenting with numerous low-voltage phosphors for fieldemission device applications. The availability of low voltage phosphorsthat have efficient excitation and voltages of 100-200 Volts wouldenhance the performance of a flat-screen display according to thisinvention. However, it is contemplated that existing high-efficiencyphosphors can be utilized according to this invention.

Since the gain of the channel 118 is in the range of 100 to 200, for agiven anode current the cathode current will be approximately the samefactor of 100 to 200 lower than without the presence of the microchannelelement 114. Any excess current flowing through the microchannel element114 that does not contribute to anode current is considered parasitic.However, designing a current saturation point into the microchanneloutput to enhance uniformity serves to lower the excess current andassures minimum dissipation. Excess saturation should be avoided if itis desired to display a gray scale or color where variable brightness(due to variable electron current) is required.

As discussed above, the flat panel display according to this inventioncomprises a large array of cathode emitters 102 with correspondingmicrochannels 118 each directed to a portion of a phosphor element 130on a screen 132. Each pixel (which can be composed of a single emitteror a plurality of parallel-connected emitters) is individuallyaddressable. For an exemplary ten inch diagonal full-color displayhaving VGA resolution, the following parameters are contemplated:

Display area: (10 inch diagonal): 280 square centimeters;

Pixel pitch (at most) 300 microns between each pixel;

Pixel area: 9×10⁻⁴ square centimeters:

Number of pixels: 300,000 (monochromatic);

Number of addressable subpixels: 900,000;

Number of cathodes for each subpixel: 3 (redundancy for

enhanced reliability); and

Total number of cathodes and microchannels: 2700,000.

For an average screen (anode) current of 1 milliamp, the averagecurrent-per-cathode, in the absence of microchannel plate would be 370picoamps (1 milliamp/2,700.000). Using, a microchannel plate, biased toagain of 100, the average,C current-per-cathode would be reduced to 3.7picoamps (generally within an order of magnitude between 1-10 picoamps),with a corresponding reduction in driver voltage. The net power isapproximately 1 watt (1 milliamp ×1000 Volts). In comparison, abacklighted active matrix liquid crystal digital display (AMLCD)consumes up to eight watts. Most of this power consumption isattributable to the fluorescent backlight.

With further reference to FIGS. 5 and 6, a basic addressing scheme for adisplay according to this invention is disclosed. To address any numberof pixels in a dynamic manner, the timing of voltages applied to thecathode, gate, microchannel plate an anode must be controlled.Generally, the anode and microchannel plate can be held at constantvoltages that are positive with respect to the gate and cathode. Thegate and cathode are modulated to control cathode current. Withreference again to FIG. 7, the voltages at each of the cathode 106, gate108, conductive dynode surfaces of the microchannel plate 120 and 122and anode 128 are V_(C), V_(G), V_(D1), VD_(D2) and V_(A) respectively.The voltage scheme is generally V_(A)>V_(D2)>V_(D1)>V_(G)>V_(A), withV_(A), V_(D1), and VD₂ held constant V_(G) and V_(C) variable in timefor the purpose of addressing individual pixels. As noted above,individual color pixels consist of three subpixels, one each for red,green or blue according to this embodiment. Additionally, each subpixelmay consist of many emitters, each with a corresponding microchannelelement. These redundant emitters within a subpixel are, typically,electrically connected in parallel. In a monochromatic display,subpixels are not normally included, but each pixel can comprise manyemitters in parallel as in a color display. Alternately, clusteredemitters could be each separately addressable to control the brightnessof a given pixel by, for example, activating a given number of clusteredemitters at a given time to generate a corresponding brightness levelbased upon the number emitters.

As depicted, the array comprises a series of interconnected rows (m) andcolumns (n) of emitters 102 and gates 108 that are each individuallyaddressable. As noted above, it can be assumed that a multiplicity ofemitters can be connected in parallel for any given row (m) or column(n) subpixel. The total number K of rows and total number Q of columnsvary depending upon the density of pixels and size of the display.

With reference to FIG. 6, at time t=0 the raster of rows (m) isinitiated by applying a voltage to the cathodes common to the first rows(m=1) for a duration T1.

Any emitter 102 in the row can then be activated by applying a voltageduring the same time interval to the intersecting gate column (n)corresponding to the emitter to be activated. The voltage differenceV_(G)−V_(C) at the emitter 102 must exceed the threshold voltagenecessary to initiate the flow of current from the cathode. Note thatthere will also be a small but significant contribution to the net fieldat the cathode from voltages applied to the microchannel plate 114 andthe anode 128. Thus, to activate an emitter 102 at any giving rastercycle, there must be present at that time an adequate field at thecathode derived from the applied voltages, especially those at thecathode and gate 108.

The timing for addressing an individual emitting element depends partlyupon the properties of the phosphor 130. In particular, the persistenceof the phosphor 130 will determine the raster rate at which theindividual subpixels must be refreshed in order to avoid flicker in thedisplay. Thus, T1 must be less than the duration of the raster cycle butsufficiently long that the intensity of a subpixel does not appear todecrease before it is refreshed on the subsequent raster scan. If thesubpixel is not to be addressed on the subsequent raster scan, however,T1 in combination with the persistence of the phosplhor column must notbe so long as to produce “ghost” images.

With reference, finally, to FIG. 8, there is shown a schematic array ofsubpixels 140, 142 and 144 of a pixel 146 for use in a color display.The subpixels 140, 142, 144 are arranged in three side-by-side columns,corresponding to the primary colors red 150, green 152 and blue 154,respectively. Each subpixel column is composed of three parallelsubpixels of like color. Each subpixel can receive electrons from one ormore cathode emitters. It is contemplated that a larger or smallernumber of subpixels can be utilized and that the subpixels can beorganized in rows, columns or another clustering arrangement (such astriangles) relative to each other. The screen phosphor should beprocessed and aligned in a manner that corresponds with the desiredpixel color. In this embodiment, each pixel (146) has a pitch relativeto other pixels (not shown) of approximately 300 microns. The pitchbetween subpixels is approximately 90 microns.

FIGS. 9-11 detail an embodiment of this invention for use in electronbeam lithographic processes used, for example, in the manufacture ofsemiconductor microcircuit chips. FIG. 9 details, in side view, ageneralized arrangement for an electron beam lithography device 200 witha microchannel plate 202 to provide gain to a cold cathode structure 204according to an embodiment of this invention. The cold cathode structure204, in particular, comprises an emitter 206 (shown as a generalizedpoint) having one or more individual cathode tips, adjacent to arespective plate channel 208. The emitter 206, as noted above, can be asingle tip with respect to a given channel, or can comprise several tipsin a cluster, aligned with the respective channel 208. In a preferredembodiment, 30-40 tips can comprise a single “emitter” aligned withrespect to a given channel 208. As will be described further, there isprovided a large number of cathodes and respective channels arranged ina two-dimensional array to contemporaneously produce a large number ofindividual, amplified electron beams over a given area. All aregenerally powered by a similar voltage V_(C). The cathodes can beindividually powered as an option.

A gate or “grid” 210 is provided between the cathode structure 204 andthe input side 212 of the microchannel plate 202. Grid openings 214,that are independently addressed, modulate emissions of electrons fromeach emitter 206 into a respective channel 208. The structure andfunction of the grid 210 and the microchannel plate 202 are inaccordance with the principles already described above. Briefly a pairof dynode plates or sides 220 and 222 are located at the respectiveinput face and output face of the microchannel plate 202. The sides aredriven at respective voltages V_(D1), and V_(D2) to define acorresponding voltage differential across the microchannel plate 202. Anelectron-emissive layer 224, as also described above is provided to theinner surface each channel, which is typically cylindrical,substantially along the entire length of the channel, the channel causesa massive gain by producing a cascade of secondary electrons for each ofelectron striking the channel—the striking electrons being from eitherthe cathode emitter 206 itself, or from a downstream rebound bysecondary electrons (see FIG. 3).

Electrons exiting each channel 208 pass thorough an electrostatic lensstructure 230 that comprises a series of three parallel lens plates232(1), 232(2) and 232(3) in this embodiment. Each of the lens plateshas, in turn, three stacked elements each. The number of lens plates andelements within the plates can be varied, and the number shown should betaken only by way of example. The lens plates each have respectiveapertures 234(1), 234(2) and 234(3) aligned with the outlet of arespective channel 208. Each lens 232(1), 232(2) and 232(3) is driven ata respective driving voltage V_(L1), V_(L2) and V_(L3).

The overall lens “stack” shown in FIG. 9 is a schelatic representationof a typical electrostatic focusing system. In almost all cases the lensstack is comprised of metal lens elements each having apertures throughwhich the beam passes. As the electron beam emerges from the channel 208it is drawn by a more-positive voltage into aperture 234(1) of the firstlens 232(1). In this embodiment, each microchannel is aligned with acorresponding set of lens stack apertures. The complexity and number ofelements in the lens stack are determined by any existing aberrationsthat have to be corrected in order to focus the beam to a minimum spotsize. The voltages through the stack rise and fall accordingly. Typicalaberrations that have to be corrected are related to the size of thesource that is to be imaged, the energy spread of the emerging electronsand their angular distribution. In this example, the source to be imagedis tile output end of the microchannel.

The magnitude of the voltages on the various lens elements also dependsupon the sensitivity of the electron beam resist at the anode as well asthe above cited aberrations. For a thin, low voltage resist the finalvoltage V_(L3) in the stack may be as low as 1000 V. Inside the stack,voltages may rise and fall throughout the range of 100 V to 4000 V. Inan embodiment that has been designed and simulated, the lens stack hasten elements with the following progression of voltages away from themicrochannel for one particular configuration: 100 V, 88 V, 500 V, 500V, 553 V, 2000 V, 2000 V, 2000 V, 3684 V, 1000 V, with the anode resistat V_(A)=1000 V, In addition to the voltages, the spacing of the lenselements should be closely controlled. For the above-describedembodiment, the overall height of the stack, that is, the distance fromthe microchannel to the resist target at the anode, is approximately 25mm.

The lenses focus each amplified electron beam into a point, which can becircular in cross-section, having a diameter of less than 100 nm. Thepoint beam, in this embodiment is used to strike a portion of asemiconductor substrate 240, having a resist layer 242 that is alteredby the beam, Hence, in this embodiment, the substrate 240 comprises theanode that absorbs the beams electrons. It is maintained at a voltagepotential V_(A), that has a value typically in accordance with thefactors described above. The resist layer 242 can be any acceptable thinfilm coating composed of polymers or other compounds that reactsselectively to a concentrated electron beam without substantialspreading of the exposure effect beyond the directly affected area. Thelayer 242 can either be a substance that undergoes a chemical changethat allows selective etching in a subsequent step, or a substance thatis actually removed by the electron beam in selected areas to reveal thesemiconductor surface below for further processing.

FIG. 10 details a small portion of an electron beam lithography array,implementing the elements described in FIG. 9, is shown in plan viewlooking downwardly from the output of the lenses. In particular, acorner of an array of multiple rows and multiple columns of beamgenerators (as the component layers/plates will be collectively termed)is shown. Rows extend in the direction of arrow Y, while columns extendin the direction of arrow X. The number of rows and columns can varydepending upon the size of the overall array and the number of separatebeams (and the density of beams) being employed. The spacing betweenbeam generators is also variable and depends partly upon how closely thegenerators can be placed from one another in fabrication. In thisembodiment row address pads 250, connected with an address controller(not shown) provide driving voltage to the cathode clusters 206 throughcathode address lines 260 formed on the cathode plate. Likewise, columnaddress pads 264 interconnect a grid address controller (not shown) withthe appropriate grid section for each cathode cluster. Note, theperimeter of the grid aperture is represented by the circles 254. Gridaddress lines 266 are provided on the grid plate. The microchannel platechannels are not visible, but are sandwiched between the grid lines andthe lens structure apertures represented by the circles 262.

With further reference to FIG. 11, the complete electron beamlithography device is detailed in exploded view. Each element of thedevice comprises a separate plate that can be formed using appropriatesemiconductor fabrication techniques that are currently available. Inpractice, the entire lithography device is typically enclosed in anevacuated chamber so that electrons can pass through the device andstrike the anode substantially free of the effects of gas molecules. Inone embodiment, the cathode, or field emitter, array 300 is the firstplate in the stack. In close-up are shown the filed emitter clusterswith associated grid (302). Address pads for connecting to thecontroller are shown. The address pads enable selective operation ofparticular emitters. Each cluster is spaced by 125 microns on-center inthis embodiment. However, it is expressly contemplated that the spacingcan vary significantly in alternate implementations.

The cathode/emitter clusters 306 themselves are shown in close-up 308.Each cathode is spaced on-center at 5 microns, and thirty to forty aregrouped together in this embodiment.

Three stacked microchannel plates 310(1), 310(2) and 310(3) are used inthis embodiment. It is expressly contemplated that any number ofmicrochannel plates, with microchannels allied can be stacked togetherto further multiply the gain effect. Each microchannel plate is poweredas described above. The driving voltages may vary from plate-to-platedepending upon the application. Precise voltages can be determinedexperimentally by operating the unit to obtain the proper gain toproduce a beam of electrons having, a desired current. A close-up of amicrochannel plate surface reveals the individual microchannels 312,each having an inside diameter of 25 microns and an on-center spacing of125 microns, in alignment with associated emitter clusters.

Downstream of the microchannel plates are positioned the electrostaticlenses 320(1), 320(2), 320(3) and 320(4). Each individual lens elementis driven at a substantially constant driving voltage across itssurface, and as described above, the voltages vary from lens to lens tocreate the desired focusing (effect at the outlet. In this embodiment afocused spot size of 100 nm or less. The apertures 330 in each lens, asshown in close-up are 80 microns in diameter. Likewise the apertures 330are spaced on-center at 125 microns and are aligned with the emittersand the microchannels.

The alignment of the elements of the device can be accomplished usingconventional alignment techniques for semiconductor devices. Connectinglugs 340 are provided in the element frame pieces 344. Any acceptablehousing or connector arrangement can be employed to join the elementstogether. Likewise a variety of connectors and wires (not shown) caninterconnect each of the elements with appropriate electroniccontrollers and voltage sources in a manner clear to one of ordinaryskill. In particular, an address controller 350, comprising anyacceptable data processing device such as a high-performanceminicomputer or microcomputer used in semiconductor fabricationtechniques, is interfaced with the row and column address pads on theemitter/grid plates. The controller 350 receives a design pattern,representing a desired circuit pathway, from a data storage device, suchas the computer disc drive 352.

It is contemplated that the beams can be provided over an area as largeas the area of a typical semiconductor wafer. Accordingly, an entiresemiconductor wafer can be exposed in motions limited to the spacingbetween electron beams. Alternatively beams can be provided over asignificant area of a wafer and stepper movement, or another techniquecan be employed to move the beams about the wafer.

In operation, a design pattern for an integrated circuit is provided indata form to a controller. The controller addressees the appropriateemitter/grid structures within the device to simultaneously expose thewafer in the design pattern. The beams are operated as the resist-coatedwafer is moved relative to the electron beams. The motion of the waferrelative to the electron beams is accomplished through a precision X-Ytranslation stage of the overall lithography device. By synchronizingthe motion of the stage with the activation and deactivation of theemitter array, according to the controller's preprogrammed addressingscheme, the pattern of the integrated circuit is replicated in theelectron beam resist. Because of the highly parallel nature of theaddress scheme, the production of high-density circuits is substantiallyaccelerated in comparison to the prior art. Again, this is because theprior art depends upon serially addressing the pattern, which is slow incomparison.

The foregoing has been a detailed description of a preferred embodimentof this invention. Various modifications and additions are contemplatedwithout departing from the spirit and scope of this invention. Forexample, the principles described herein in FIGS. 5, 6 and 7, whilerelated to a flat panel display, can be applied with modifications toother imaging devices such as an electron microscope or a electron beamlithography device and vice versa. In particular any output from themicrochannel plate herein can be focused using one or more electrostaticor equivalent lenses before it strikes an appropriate anode. Theparameters of the microchannel, gate structure and cathode structurecould be altered to meet the specific needs of an electron microscopicor lithographic environment as generally described herein. Accordingly,this description is meant to be taken only by way of example and tootherwise limit the scope of the invention.

What is claimed is:
 1. A field emission device comprising: a pluralityof field emission cathodes for generating a plurality of streams ofelectrons; a gate structure positioned relative to each of the fieldemission cathodes for modulating the stream of electrons generated byeach of the field emission cathodes; a microchannel gain element having:(i) a first dynode side adjacent the gate and an opposing, second dynodeside, wherein each of the first and second dynode sides includes aconductive material thereon having a voltage difference therebetween,and (ii) a plurality of microchannels each having a secondaryelectron-emissive layer therein and each located adjacent each of theplurality of field emission cathodes for providing a gain to each of thestreams of electrons; and an anode positioned adjacent the second dynodeside, the anode absorbing electrons from each of the plurality ofstreams of electrons.
 2. The field emission device as set forth in claim1, wherein the anode comprises a transparent screen having phosphorthereon.
 3. The field emission device as set forth in claim 1, whereinthe microchannel gain element comprises a plate having a substrateconstructed from glass.
 4. The field emission device as set forth inclaim 1, wherein the microchannel gain element comprises a substrateconstructed from silicon.
 5. The field emission device as set forth inclaim 4, wherein the microchannel gain element includes an insulatinglatter located thereon and alone a surface of each of the microchanneland wherein each of the first dynode side and the second dynode side areinsulated from the substrate by the insulating layer and furthercomprising a resistive bridge layer interconnecting each of the firstdynode side and the second dynode side and the resistive bridge layerbeing insulated from the substrate by the insulating layer.
 6. The fieldemission device as set forth in claim 1, wherein the microchannel gainelement comprises a substrate constructed from a metal.
 7. The fieldemission device as set forth in claim 1 further comprising anelectrostatic lens structure located between the microchannel rainelement and the anode for focusing each of the plurality of streams ofelectrons.
 8. The field emission device as set forth in claim 1, whereinthe anode comprises a semiconductor substrate having a resist layer thatis selectively acted upon in a localized region thereof by the pluralityof streams.
 9. The field emission device as set forth in claim 8,further comprising an address controller interconnected with at leastone of the field emission cathodes and the gate structure that controlsemission of electrons from the cathodes selectively according to apredetermined circuit design pattern whereby a corresponding pattern isproduced the resist layer.
 10. The field emission device as set forth inclaim 1, wherein the plurality of field emission cathodes comprisegroupings that correspond to a plurality of pixels and wherein the anodecomprises a display for displaying the pixels.
 11. The field emissiondevice as set forth in claim 10, wherein the display is constructed andarranged to display at least 300,000 pixels.
 12. The field emissiondevice as set forth in claim 1, wherein each of the first dynode sideand the second dynode side of the microchannel lain element are drivenat a predetermined voltage and wherein a difference between thepredetermined voltage oil each of the first dynode side and the seconddynode side is in a range between approximately 600 and 1000 Volts. 13.The field emission device as set forth in claim 1, comprising at least2,000,000 field emission cathodes.
 14. The field emission device as setforth in claim 1 wherein each of the cathodes generates a current of theorder of 1 picoamp.
 15. The field emission device as set forth in claim1 wherein each of the cathodes comprises a cluster of a plurality ofcathode tips all aligned with a predetermined microchannel of theplurality of microchannels.
 16. The field emission device as set forthin claim 1, wherein the plurality of field emission cathodes comprisegroupings that correspond to electron beams used in a lithographicprocess and wherein the anode comprises a resist layer that isselectively exposed by the electron beams.
 17. A method for inducing acurrent gain in a stream of electrons emitted by a field emission devicecomprising the steps of: emitting a stream of electrons having a firstcurrent from a field emission device; directing the stream of electronshaving the first current into a microchannel of a microchannel gainelement, the microchannel gain element having: (i) a first dynode sideand an opposing, second dynode side, wherein each of the first andsecond dynode sides includes a conductive material thereon having avoltage difference therebetween, and (ii) a plurality of microchannelseach having a secondary electronemissive layer therein and each locatedadjacent a field emission device; applying a driving voltage to themicrochannel in which the stream of electrons having the first currentis directed to generate a resulting stream of electrons that exits themicrochannel having a second current that is greater than the firstcurrent; and striking an anode with the resulting stream of electronshaving the second current.
 18. The method as set forth in claim 17,wherein the step of striking includes exciting with the resulting streamof electrons, a visible light emission from a phosphor located on theanode.
 19. The method as set forth in claim 18, wherein the step ofexciting includes exciting a phosphor that emits a substantially greatervisible light in response to contact by the resulting stream ofelectrons having the second Current than a visible light emission by thephosphor in response to contact by a stream of electrons having thefirst current.
 20. The method as set forth in claim 17, wherein the stepof striking includes exposing a resist layer on a lithographicsubstrate.
 21. The method as set forth in claim 20, wherein the step ofexposing includes focusing the stream with an electrostatic lensstructure positioned between the microchannel gain element and thesubstrate.
 22. The method as set forth in claim 21, further comprising,directing a plurality of streams of electrons through discretemicrochannels of the microchannel gain element, the step of directingfurther including selectively addressing a plurality of field emissiondevices to according to a selected two-dimensional pattern to produce apredetermined exposure pattern on the substrate.
 23. The method as setforth in claim 22, further comprising moving the substrate relative tothe streams of electrons between exposure passes to expose adjacentlocations on the substrate between previously exposed portions whereby acomplete exposure pattern is created on the substrate.
 24. A fieldemission device adapted for use in a lithographic process, the fieldemission device comprising: a plurality of field emission cathodes forgenerating a plurality of streams of electrons; a gate structurepositioned relative to each of the field emission cathodes formodulating the stream of electrons generated by each of the fieldemission cathodes; a microchannel gain element having: (i) a firstdynode side adjacent the gate and an opposing, second dynode side,wherein each of the first and second dynode sides includes a conductivematerial thereon having a voltage difference therebetween, and (ii) aplurality of microchannels each having a secondary electron-emissivelayer therein and each located adjacent each of the plurality of fieldemission cathodes for providing a gain to each of the streams ofelectrons; and an anode positioned adjacent the second dynode side, theanode adapted to facilitate exposure of a resist layer by the pluralityof streams of electrons.
 25. The field emission device as set forth inclaim 24, further comprising an electrostatic lens structure locatedbetween the microchannel gain element and the anode for focusing each ofthe plurality of streams of electrons.
 26. The field emission device asset forth in claim 24, wherein at least one of the field emissioncathodes and the gate structure are adapted to be interconnected with anaddress controller that controls emission of electrons from the cathodesselectively according to a predetermined circuit design pattern wherebya corresponding pattern is produced in the resist layer.